The natural intervertebral disc contains a jelly-like nucleus pulposus surrounded by a fibrous annulus fibrosus. Under an axial load, the nucleus pulposus compresses and radially transfers that load to the annulus fibrosus. The laminated nature of the annulus fibrosus provides it with a high tensile strength and so allows it to expand radially in response to this transferred load.
In a healthy intervertebral disc, cells within the nucleus pulposus produce an extracellular matrix (ECM) containing a high percentage of proteoglycans. These proteoglycans contain sulfated functional groups that retain water, thereby providing the nucleus pulposus within its cushioning qualities. These nucleus pulposus cells may also secrete small amounts of cytokines such as interleukin-1.beta. and TNF-.alpha. as well as matrix metalloproteinases (“MMPs”). These cytokines and MMPs help regulate the metabolism of the nucleus pulposus cells.
In some instances of disc degeneration disease (DDD), gradual degeneration of the intervertebral disc is caused by mechanical instabilities in other portions of the spine. In these instances, increased loads and pressures on the nucleus pulposus cause the cells within the disc (or invading macrophages) to emit larger than normal amounts of the above-mentioned cytokines. In other instances of DDD, genetic factors or apoptosis can also cause the cells within the nucleus pulposus to emit toxic amounts of these cytokines and MMPs. In some instances, the pumping action of the disc may malfunction (due to, for example, a decrease in the proteoglycan concentration within the nucleus pulposus), thereby retarding the flow of nutrients into the disc as well as the flow of waste products out of the disc. This reduced capacity to eliminate waste may result in the accumulation of high levels of toxins that may cause nerve irritation and pain.
As DDD progresses, toxic levels of the cytokines and MMPs present in the nucleus pulposus begin to degrade the extracellular matrix, in particular, the MMPs (as mediated by the cytokines) begin cleaving the water-retaining portions of the proteoglycans, thereby reducing its water-retaining capabilities. This degradation leads to a less flexible nucleus pulposus, and so changes the loading pattern within the disc, thereby possibly causing delamination of the annulus fibrosus. These changes cause more mechanical instability, thereby causing the cells to emit even more cytokines, thereby upregulating MMPs. As this destructive cascade continues and DDD further progresses, the disc begins to bulge (“a herniated disc”), and then ultimately ruptures, causing the nucleus pulposus to contact the spinal cord and produce pain.
One proposed method of managing these problems is to remove the problematic disc and replace it with a porous device that restores disc height and allows for bone growth therethrough for the fusion of the adjacent vertebrae. These devices are commonly called “fusion devices” or “fusion cages”.
Current interbody fusion techniques typically include not only an interbody fusion cage, but also supplemental fixation hardware such as fixation screws. This hardware adds to the time, cost, and complexity of the procedure. It also can result in tissue irritation when the cage's profile extends out of the disc space, thereby causing dysphonia/dysphagia in the cervical spine and vessel erosion in the lumbar spine. In addition, the fixation hardware typically includes a secondary locking feature, which adds to the bulkiness of the implant and time required for the procedure. Furthermore, existing fixation hardware may prevent the implantation of additional hardware at an adjacent location, and so require removal and potentially extensive revision of a previous procedure.
US Published Patent Application 2008-0312698 (Bergeron) discloses a device and system for stabilizing movement between two or more vertebral bodies and methods for implanting. Specifically, the embodiments provide medical professionals with the ability to selectively position and orient anchors in bony tissue and then attach a plate to the pre-positioned anchors. The plate assembly, once positioned on the anchors, prevents the anchors from backing out of the bony tissue. Furthermore, in situations in which it is desirable to provide spacing between two vertebral bodies, a spacer may be fixedly connected to the plates for positioning between two vertebral bodies. The spacer may further function as a lock out mechanism, or may be rotatably connected to the plates to maintain rotational freedom. The spacer may incorporate connection features or attachment features.
U.S. Pat. No. 4,904,261 (Dove) discloses a spinal implant, e.g., to replace an excised disc, comprising a rigid generally horseshoe shape of biocompatible material, such as carbon-fibre reinforced plastics, having upper and lower planar faces converging towards the ends of the horseshoe, and at least one hole from each planar face emerging in the outer curved face of the horseshoe, to enable the horseshoe to be fixed by screws inserted through one or more selected holes in each plurality from the ends in the outer curved face into respective adjacent vertebrae, with the screw heads bearing against shoulders, and with the space bounded by the inner curved face of the horseshoe available for the insertion of bone graft or a bone graft substitute.
U.S. Pat. No. 6,579,290 (Hardcastle) discloses a surgical implant for fusing adjacent vertebrae together comprising a body portion with spaced arms. The body portion has passages to receive surgical fixing screws engaged in holes drilled in the vertebrae for securing the body portion to the anterior faces of the vertebrae to be fused. The arms extend into a prepared space between the vertebrae to be fused. Graft material is packed between the arms. Each surgical fixing screw has an externally screw-threaded shank divided into wings which can be outwardly deformed to anchor the shank in the hole. Each surgical fixing screw also has a head which can be transformed between a laterally expanded condition and a laterally contracted condition to permit the head to be interlocked with the implant.
U.S. Pat. No. 6,342,074 (Simpson) discloses a spinal fusion implant and method for maintaining proper lumbar spine curvature and intervertebral disc spacing where a degenerative disc has been removed. The one-piece implant comprises a hollow body having an access passage for insertion of bone graft material into the intervertebral space after the implant has been affixed to adjacent vertebrae. The implant provides a pair of screw-receiving passages that are oppositely inclined relative to a central plane. In one embodiment, the screw-receiving passages enable the head of an orthopedic screw to be retained entirely within the access passage. A spinal fusion implant embodied in the present invention may be inserted anteriorly or laterally.
U.S. Pat. No. 6,972,019 (Michelson) discloses a spinal fusion implant for insertion between adjacent vertebral bodies that has opposed upper and lower surfaces adapted to contact each of the adjacent vertebral bodies from within the disc space, a leading end for insertion between the adjacent vertebral bodies, and a trailing end opposite the leading end. The trailing end has an exterior surface and an outer perimeter with an upper edge and a lower edge adapted to be oriented toward the adjacent vertebral bodies, respectively, and a plurality of bone screw receiving holes. At least one of the bone screw receiving holes is adapted to only partially circumferentially surround a trailing end of a bone screw received therein. At least one of the bone screw receiving holes passes through the exterior surface and one of the edges so as to permit the trailing end of the bone screw to protrude beyond one of the edges.
US Patent Publication 2009-0030520 (Biedermann) discloses a fixation device for bones that includes a member which is to be fixed to one or more bones and has at least one bore for receiving a bone screw, wherein the at least one bore comprises a first internal thread portion. The bone screw has a first shaft section provided with a first external thread portion arranged to cooperate with the internal thread portion of the at least one bore, and a head section having a diameter larger than that of the shaft section to provide a catch arranged to engage with a stop formed in the bore. The bone screw further has a second shaft section which includes a clearance groove extending between the catch of the head section and the external thread of the first shaft section. The clearance groove allows disengagement of the two thread portions, such that the bone screw is prevented from being unscrewed off the bore when it is loosened within the adjacent bone. The member can also include a side wall of a cage used in an intervertebral implant device, or can represent a plate of a bone plate assembly.